This page explains how to calculate the pulse duration of a monostable multivibrator using a 555 timer IC. Just put the resistance and capacitance values, and it will show the output pulse width (Tp) in seconds.
As per a standard IC 555 calculation law, it is advised to select the resistor values higher than 1K and lower than 1 Meg. The Capacitor with high values of electrolytic capacitor should not be used, rather larger value R's must be opted which will ensure lower power consumption and higher accuracy of the output time delays due to lower leakage.
Calculator by homemade-circuits.com
EXAMPLE:
Inputs: Resistance = 220KΩ, Capacitance = 1µF
Output: Pulse Duration = 0.242 seconds (or 242 milliseconds)